Wavelength multiplexing optical communications systems using coherent modulation such as dual polarization quadrature phase shift keying (DP-QPSK) have been introduced to address an increase in transmission capacity in optical communications. According to DP-QPSK, the transmission capacity per wavelength is 100 Gb/s. To further increase the transmission capacity, a study has been made of optical transmission using coherent modulation techniques capable of transmitting a large amount of information, such as 16 quadrature amplitude modulation (16QAM).
Such coherent modulation techniques capable of transmitting a large amount of information require a spectral linewidth that indicates the amount of wavelength fluctuation of a laser beam emitted from a light source to be narrow. For example, 16QAM requires a spectral linewidth of 100 kHz or less. For example, as a tunable laser source of a narrow linewidth, an external-cavity laser source as depicted in FIG. 1 is proposed (see, for example, International Publication Pamphlet No. WO2007/080891). This external-cavity laser source is a combination of a semiconductor optical amplifier (SOA) 910 that serves as a gain medium, a lens 921, a tunable filter 922, and an external mirror 923. According to the external-cavity laser source structured as described above, an anti-reflective (AR) coating 911 is provided on one end face 910a of the SOA 910, and a partially reflective coating 912 is provided on the other end face 910b of the SOA 910.
Accordingly, a laser resonator (cavity) is formed by the partially reflective coating 912 provided on the other end face 910b and the external mirror 923, and the lens 921 and the tunable filter 922 are disposed in the optical path between the SOA 910 and the external mirror 923. According to this tunable laser source, a laser beam is emitted from the side of the other end face 910b of the SOA 910 on which the partially reflective coating 912 is provided. The spectral linewidth of the emitted laser beam tends to be narrower as the laser resonator becomes longer. External-cavity laser sources, for which it is easy to increase the length of the laser resonator, are suitable to reduce the spectral linewidth and have achieved spectral linewidths of 100 kHz or less.
Furthermore, according to coherent modulation, input signal light input to a receiver that serves as a coherent receiver and local oscillation light having an oscillation wavelength close to the wavelength of the input signal light are caused to interfere with each other to detect a phase modulation signal. Therefore, optical transmitter and receiver modules adopting coherent modulation employ two laser sources, namely, a laser source for output signal light and a laser source for local oscillation light. Specifically, as depicted in FIG. 2, according to optical transmitter and receiver modules adopting coherent modulation, a laser source 931 is provided in a transmitter 930, and a laser source 941 is provided in a receiver 940. A laser beam emitted from the laser source 931 is modulated in a DP-QPSK modulator 932 to be output from the transmitter 930 as an output signal. Furthermore, an input signal input to the receiver 940, together with local oscillation light emitted from the laser source 941, enters a hybrid 942, and light exiting from the hybrid 942 is detected at a light-receiving element 943. The coherent-modulation optical transmitter and receiver module having a structure as depicted in FIG. 2, however, requires two laser sources, and accordingly, is large in size. Therefore, there is a demand for coherent-modulation optical transmitter and receiver modules that are reduced in size.
From such a viewpoint, a coherent-modulation optical transmitter and receiver module using a single laser beam as depicted in FIG. 3 is proposed (see, for example, Japanese Laid-open Patent Publication No. 2007-64860). According to the coherent-modulation optical transmitter and receiver module as depicted in FIG. 3, a laser beam emitted from the laser source 931 is split into signal light and local oscillation light by a beam splitter 933. Specifically, of the laser beam emitted from the laser source 931 to be incident on the beam splitter 933, a part transmitted through the beam splitter 933 becomes signal light, which enters the DP-QPSK modulator 932 to be modulated and is output from the transmitter 930 as an output signal, and a part reflected by the beam splitter 933 becomes local oscillation light, which, together with an input signal input to the receiver 940, enters the hybrid 942. Light exiting from the hybrid 942 is detected at the light-receiving element 943.
Thus, the coherent-modulation optical transmitter and receiver module depicted in FIG. 3 does not require the laser source 941 provided in the receiver 940 of the coherent-modulation optical transmitter and receiver module depicted in FIG. 2, and accordingly, can be reduced in size. According to the coherent-modulation optical transmitter and receiver module depicted in FIG. 3, however, the single laser source 931 is required to emit a laser beam that becomes both signal light and local oscillation light, and accordingly, is required to be capable of emitting a high-power laser beam.
As a laser source capable of emitting a high-power laser beam, a laser source that further includes an SOA provided in the stage subsequent to a laser resonator is proposed (see, for example, International Publication Pamphlet No. WO2007/080891). As depicted in FIG. 4, this laser source is a combination of an SOA integrated device 950, the lens 921, the tunable filter 922, and the external mirror 923. The SOA integrated device 950 includes a first SOA 951, a second SOA 952, and a partially reflecting mirror 953 formed between the first and second SOAs 951 and 952. Furthermore, an AR coating 954 is provided on one end face 950a of the SOA integrated device 950 on the external cavity side, and an AR coating 955 is provided on the other end face 950b of the SOA integrated device 950 through which a laser beam is emitted.
According to the laser source having the above-described structure, a laser resonator is formed by the partially reflecting mirror 953 of the SOA integrated device 950 and the external mirror 923. The lens 921, the tunable filter 922, and the first SOA 951 are disposed in the optical path between the partially reflecting mirror 953 and the external mirror 923. A laser beam transmitted through the partially reflecting mirror 953 is amplified by the second SOA 952 serving as an amplifier. Accordingly, the laser source having the above-described structure can emit a high-power laser beam.